Being susceptible to damage, most plastic lenses are provided with hard coating(s) by applying a silicon-containing primer or other hard-coating primer to their surface(s) and curing the primer with ultraviolet light.
Further, plastic lenses are sometimes coated with anti-reflection films formed by vapor deposition of an inorganic substance on their surface for preventing surface reflection. However, these lenses have low impact resistance and have a defect that they break in a falling ball test (FDA standards, U.S.A.).
For overcoming the above problem, there is a technique of interposing a primer layer formed of a urethane resin between a plastic lens substrate and a hard coating (see JP-A-5-25299).
Meanwhile, plastic lenses whose thickness can be decreased and whose lens substrate has a high refractive index, specifically a refractive index of 1.54 to 1.67, are widely available in recent years.
A silicon-containing hard coating generally has a refractive index of between 1.40 and 1.50, but since it is usual that the hard coating is coated by dip coating, a non-uniformity is caused in a coating thickness and difference in the coating thickness is liable to appear as interference fringes.
For overcoming the above problem, a hard coating having a high refractive index has been developed (see JP-A-5-2102, JP-A-5-164902 and JP-B-6-102776).
However, although the interference fringes can be decreased by forming a hard coating having a high refractive index directly on a lens substrate having a high refractive index, there is a defect that when an anti-reflection film is further formed thereon, the impact resistance of the lens decreases.
The lens can be improved in impact resistance by forming the above primer layer formed of a urethane resin, but there is another defect that interference fringes occur in this case because the primer layer has a refractive index of 1.50 to 1.52.
Recently, the following four laid-open publications disclose primers which improve a lens in impact resistance and also give a cured film having a high refractive index.
JP-A-4-366801 discloses a plastic lens obtained by laminating a primer layer and a hard coating layer, which satisfy the following two formulae, and a mono- or multi-layered anti-reflection layer formed by vapor deposition of an inorganic substance on the surface of a plastic lens substrate having a refractive index n.sub.s of 1.50 to 1.70 consecutively in the above order, the said primer layer being a polyurethane containing at least one member selected from halogen atoms excluding fluorine and a sulfur atom and having a refractive index n.sub.p of 1.45 to 1.60. ##EQU1## wherein n.sub.s and n.sub.p are as specified above and n.sub.H is a refractive index of the hard coating. ##EQU2## wherein d is a thickness of the primer layer and .lambda. is a wavelength of visible light which is between 450 and 650 nm.
JP-A-5-142401 discloses a plastic lens which is different from the above plastic lens disclosed in JP-A-4-366801 only in that the primer layer is formed of a compound of at least one metal selected from the group consisting of Al, Ti, Zr, Sn and Sb and polyurethane.
JP-A-6-82604 discloses an optical element formed by laminating a silicone cured coating and an inorganic anti-reflection film on the surface of an organic glass, wherein a primer layer is interposed between the said organic glass and the cured film of cured silicone, the primer layer being formed of a coating composition comprising, as essential effective components, the following components: (a) urethane elastomer, (b) inorganic fine particles and (c) hydrolyzate of organoalkoxysilane.
JP-A-6-118203 discloses a plastic lens obtained by forming a hard coating on the surface of a lens substrate of a plastic through a primer layer, wherein the primer layer is formed of a dispersion of 5 to 100 parts by weight of colloidal metal oxide fine particles having a particle diameter of 4 to 150 nm in 100 parts by weight of a heat-curable urethane resin, the said colloidal metal oxide fine particles containing 60 to 99% by weight of titanium oxide.
Further, JP-A-6-337379 discloses a plastic lens obtained by forming a primer layer as an impact-absorbing layer and a scratch-resistant hard coating layer on at least one surface of a plastic lens substrate in the above order from the substrate side, the said plastic lens substrate and the said primer layer having nearly equivalent refractive indices.
However, the primer layers disclosed in JP-A-4-366801 and JP-A-5-142401 have a refractive index lower than about 1.57, and when these are formed on a lens having a refractive index of 1.60, interference fringes remain.
In the primers containing inorganic fine particles, disclosed in JP-A-6-82604 and JP-A-6-118203, the dispersing of the inorganic fine particles has its own limit, the primer layers have a low refractive index, and when these are formed on a lens having a refractive index of 1.60, interference fringes remain.
In the primer layer disclosed in the last JP-A-6-337379, it is necessary to control the primer layer so as to have a refractive index nearly equivalent to that of a plastic lens substrate, and therefore, extreme difficulty is practically involved.